The determinants of respiratory rate during mechanical ventilation

The independent and interactive effect of feedback related to volume, CO₂, inspiratory flow, and arousal state on the regulation of respiratory rate in mechanically ventilated humans is not well characterized. We examined the rate response of eight normal volunteers during both quiet wakefulness and non-rapid-eye-movement (NREM) sleep, while mechanically ventilated through a nasal mask in an assist/control mode with a machine back-up rate of 2 breaths/min. Tidal volume (VT) was set slightly above spontaneous VT and then increased by 0.2 L every 3 min up to 1.8 L or 25 ml/kg. Either an inspiratory flow of 40 L/min or an inspiratory time of 2 s (iso-TI) was set, with CO₂ added (FI(CO₂) > 0) or FI(CO₂) = 0. Measurements were made during both quiet wakefulness and NREM sleep. We found that as VT increased, the respiratory rate decreased; the rate decline was observed during wakefulness and sleep, and under isocapnic as well as hypocapnic conditions. Increasing inspiratory flow raised the respiratory rate during wakefulness and NREM sleep. During NREM sleep, hypocapnia resulted in wasted ventilator trigger efforts. In summary, both VT and inspiratory flow settings affect the respiratory rate, and depending on state, can affect CO₂ homeostasis. Ventilator settings appropriate for wakefulness may cause ventilatory instability during sleep.